C-PAK Industries offers product design assistance to all of our customers, this way we can ensure our customers get the best design possible while making sure their product is still cost effective. We work out of Solidworks, so we can accept most CAD file extensions.

Keep these 6 Design Features in mind when designing your product

Sharp Edges

Radii should be added to all edges of Rotationally Molded parts as often as possible for 2 reasons, 1) An air gap could form between the sharp exterior edge of your part and the material causing a visible void to form on your part, and 2) They distribute any corner stress on a part over a broader area and add to the strength of your part. The larger the radius the better, but a minimum radius of .125” is recommended for all parts. Adding radii will also minimize the increases and decreases of the wall thickness at the corners, creating a more uniform nominal wall thickness throughout your part.

Wall Separation

Considering the nature of the Rotational Molding process, you must provide an adequate distance between parallel walls to allow the material to come into contact with all surfaces of the mold. If parallel walls are too close to each other, it will cause the material to bridge over the area between the walls and prevent the rest of the mold cavity from filling completely. This in turn will cause either the wall thickness to be extremely thin in the area beyond where the material bridged off, or will cause features on your part to not form at all. The rule of thumb is to allow for a parallel wall separation of 5x the wall thickness, however, it is possible to get away with a wall separation of 3x the wall thickness in areas. Contact our experienced engineering staff to discuss this further for your product.

Flat Walls

Flat Walls are hard to produce in Rotational Molding. Since Rotationally Molded parts are formed in hollow molds with no internal cores, it makes it difficult to guarantee flatness. A typical flatness tolerance in Rotational Molding is between +/- .020 – +/- .050” per inch. If a flat surface cannot be avoided, they should at the very minimum be supported with reinforcement ribs or kiss offs. Another viable option for a flat wall would be to add a crown to the surface at .015” per inch for large surfaces.

Draft Angles

How much draft you add to your part is really dependent on the design of your part. Draft Angles are primarily used to make it easier to remove the molded part from the mold cavity. Some designs, such as a cylinder, may not need draft added. This is due to the fact that these hollow parts are molded without internal cores. As the plastic is cooled it will shrink away from the mold wall. This is not the case with all part designs though. A donut shaped part, although cylinder in shape, would need draft added. As this part cools, it would shrink away from the mold wall, however, the surface that forms the hole in the center would shrink down tightly against the mold. Removal of the part would be made easier by providing sufficient draft angles on these surfaces. A minimum of 1 Degree draft should be added to all parts, but in some designs a larger draft angle may be needed.


One of the advantages of Rotational Molding is the ability to design Undercuts into your product. An Undercut by definition is “any inwardly or outwardly projecting wall that is parallel to the parting line of the mold which must be deformed in order to be removed from the mold.” Since this process molds hollow parts with no inside core, it does allow the walls of the part to be shifted inwardly during demolding to free an undercut. However, in most cases, an additional mold section would be created during the tooling phase to accommodate the undercut so that the part would not have to be deformed during demolding. Contact our experienced engineering staff for more information on Undercuts or to have them review your current design.

Molded-In Items

Rotational Molding offers the ability to Mold-In different items directly into your plastic part. Whether you are looking to Mold-In a multi-colored graphic, embossed logo or threaded inserts, it can be done in this process. Mold-In-Graphics are good for colored logos and warning labels. Keep in mind though that these graphics are flush against the wall of the part, they are not raised. You do not have to worry about them coming off as they do not scratch off the part wall. Embossed Logos are engraved directly into the mold wall itself so it will mold on each part every time. Embossed logos will be the same color as your part color. Threaded Inserts are molded directly into the plastic part, not placed in the part post molding like some other processes. This gives them exceptional pull out strength. Threaded Inserts come in Standard, Metric and NPT sizes, in either Aluminum, Brass or Stainless Steel.


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